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Topic: Do orbiting bodies accelerate? (Read 1915 times)

This is just a series of thoughts leading to a “conclusion”. I would appreciate comments/criticism, please.

1. Velocity is a vector which includes speed and direction.2. Acceleration is change in velocity.3. Change of speed with constant direction = acceleration.4. Change of direction with constant speed = acceleration.5. A body orbiting at constant speed is constantly accelerating.6. Gravity is not a force that holds an orbiting body as though it were on a string.7. Gravity alters the geometry of spacetime such that it becomes (or acts as though) curved.8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.10. A body travelling at constant speed in a straight line in flat spacetime is not accelerating.11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.

This is just a series of thoughts leading to a “conclusion”. I would appreciate comments/criticism, please.

1. Velocity is a vector which includes speed and direction.2. Acceleration is change in velocity.3. Change of speed with constant direction = acceleration.4. Change of direction with constant speed = acceleration.5. A body orbiting at constant speed is constantly accelerating.

All above are correct.

Quote from: Bill S

6. Gravity is not a force that holds an orbiting body as though it were on a string.

Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.

Quote from: Bill S

7. Gravity alters the geometry of spacetime such that it becomes (or acts as though) curved.

It has the ability to do that but what part of the spacetime is curved and what isn't depends on the distribution of matter. See as an example a sphere having uniform density which then has a spherical cavity cut out of it where the center of the sphere is not at the center of the spherical body. : http://home.comcast.net/~peter.m.brown/gr/grav_cavity.htm

Quote from: Bill S

8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.

Wrong. You're confusing the curvature of a worldline with the curvature of a manifold. You can have a manifold which is flat in which you can choose spacetime coordinates so that the geodesic is curved.

Quote from: Bill S

9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.

Wrong. A geodesic is a curve having extremal proper time.

Quote from: Bill S

10. A body travelling at constant speed in a straight line in flat spacetime is not accelerating.

Correct.

Quote from: Bill S

11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.

Correct.

Quote from: Bill S

12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.

Wrong. The 4-acceleration is zero. The spatial acceleration is non-zero.

Kepler's 1st law states that planetary orbits are elliptical, and his 2nd law involves a constant change of speed, so I guess using the sort of generalised model of constant speed and circular orbit is on to a hiding to nothing.

Kepler's 1st law states that planetary orbits are elliptical, and his 2nd law involves a constant change of speed, so I guess using the sort of generalised model of constant speed and circular orbit is on to a hiding to nothing.

Quote from: Pete

The 4-acceleration is zero. The spatial acceleration is non-zero.

Could you explain this in simple, layman, terms, please?

Sure. 4-acceleration is an acceleration which depends on the mass of the object. Spatial acceleration is the time rate of change of a particle/bodies position with time.

Kepler's 1st law states that planetary orbits are elliptical, and his 2nd law involves a constant change of speed, so I guess using the sort of generalised model of constant speed and circular orbit is on to a hiding to nothing.

Quote from: Pete

The 4-acceleration is zero. The spatial acceleration is non-zero.

Could you explain this in simple, layman, terms, please?

The circle is just a special case of an ellipse with both focal points at the same place.

Sure. 4-acceleration is an acceleration which depends on the mass of the object. Spatial acceleration is the time rate of change of a particle/bodies position with time.

Does 4-acceleration not involve change in position with time?

Quote from: Jeffrey

The circle is just a special case of an ellipse with both focal points at the same place.

Would it not be true that Kepler’s 2nd law, applied to a circular orbit, would not involve a change of speed?

The angular velocity is constant, however objects tend to move in a straight line unless acted upon by a force. Since the direction is constantly changing due to the force of gravity you can argue that this is in fact a type of acceleration. There is an equilibrium in the kinetic energy of the orbiting body due to its initial velocity balancing with the gravitational force that is affecting its motion. Increase or decrease this critical velocity and the profile of the orbit becomes either an elliptical one or the object crashes to the surface.

The angular velocity is constant, however objects tend to move in a straight line unless acted upon by a force. Since the direction is constantly changing due to the force of gravity you can argue that this is in fact a type of acceleration. There is an equilibrium in the kinetic energy of the orbiting body due to its initial velocity balancing with the gravitational force that is affecting its motion. Increase or decrease this critical velocity and the profile of the orbit becomes either an elliptical one or the object crashes to the surface.

Agreed, but doesn't that beg the question as to whether an object in orbit is being held there by the force of gravity, or is simply following the most direct path in curved spacetime?

When GRists say that "gravity is not a force" all they mean is that it's not a 4-force.

That's not quite what David Deutsch says here:

“In the nineteenth century, few things would have been regarded more confidently as real than the force of gravity. Not only did it figure in Newton’s then-unrivalled system of laws, but everyone could feel it, all the time, even with their eyes shut – or so they thought. Today we understand gravity through Einstein’s theory rather than Newton’s, and we know that no such force exists. We do not feel it! What we feel is the resistance that prevents us from penetrating the solid ground beneath our feet. Nothing is pulling us downwards. The only reason why we fall downwards when unsupported is the fabric of space and time in which we exist is curved.”

When GRists say that "gravity is not a force" all they mean is that it's not a 4-force.

That's not quite what David Deutsch says here:

“In the nineteenth century, few things would have been regarded more confidently as real than the force of gravity. Not only did it figure in Newton’s then-unrivalled system of laws, but everyone could feel it, all the time, even with their eyes shut – or so they thought. Today we understand gravity through Einstein’s theory rather than Newton’s, and we know that no such force exists. We do not feel it! What we feel is the resistance that prevents us from penetrating the solid ground beneath our feet. Nothing is pulling us downwards. The only reason why we fall downwards when unsupported is the fabric of space and time in which we exist is curved.”

He's wrong. Just because we can't "feel" a force it doesn't mean that we're not subjected to a force. All it means is that all of the particles that make up our body accelerate at the same rate when in a uniform gravitational field. However when we're in a non-uniform gravitational field that's no longer true. The particles that make up our body accelerate at slightly different rates. It's just that the difference is too small for humans to sense the difference. The Earth's gravitational field is such field. I'm referring to tidal forces of course.

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